Electronics > Beginners
What voltage battery do I need ?
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mariush:
As for AA batteries, you have to keep in mind that user may insert rechargeable or plain non-rechargeable batteries

Alkaline non-rechargeable batteries will be up to around 1.65v when fully charged but they'll quickly drop to 1.5v and then go down to 1v
Rechargeable will be 1.35v when full and go down to around 1.1v

So let's say you design with 3 AA batteries .. you'll have a voltage range of 3v ... 4.05v / 4.95v
With 4 AA batteries, you'd have 6v with alkaline batteries, but as batteries discharge the voltage would go down quickly. So you'd still need a step-up regulator to boost 4 x 1v ... 4x1.5v to 6v
radiolistener:
I'm suspect that power consumption of step-up DC-DC may be even worse than linear LDO.

Also step-up/step-down DC-DC have extreme high noise. It may very seriously affect device functions.

While linear LDO has very clean output with no noise.

In order to reduce power wasting on regulators, you can select battery voltage very close to required voltage. So the voltage difference between LDO input and output will be minimal.
mariush:
There are switching regulators with up to 90-95% efficiency.... it may beat the losses caused by reducing 6v+ to 3.3v... again, it depends on power consumption of 3.3v device vs 6v device power consumption ...
As for noise, you can always add extra filtering (pi filters) or boost by a bit more than 6v then use a very low drop regulator to bring down to 6v
mzzj:

--- Quote from: radiolistener on September 16, 2019, 07:34:42 pm --- Also you can use just two 3V elements. You can get 3V from one element and 6V from both in series. In such way you can eliminate 3V regulator which will waste battery energy :)

--- End quote ---
Probably worth mentioning that using 2 same size batteries in series and connecting part of the load only to 3v drains one of the batteries as fast as 6v battery pack + linear regulator.
You'll just end up with one empty battery and one half-empty battery..

You could use two different size batteries or series-parallel combination but usually it is not worth the hassle.
Nobody is happy if you need 9V battery, AA battery and AAA battery for one equipment. (seen that..) 
Canis Dirus Leidy:

--- Quote from: Hextejas on September 16, 2019, 07:37:15 am ---This is always not clear to me so here I am again.
The device that I am building has 2 sub-devices.
1 needs 3.3v and the other needs 6.0v
So, do I merely sum them together = 9.3v, add in a bit of overhead, and look around for a 10v LIPO ?
Or can I use a pair of DC-DC buck converters, and use them to bring a 7.3v LIPO down to the required voltages ?

--- End quote ---
By the way. What, in fact, the requirements (size, weight, battery life, etc.) for the complete device? And then, it may turn out that one cheap 12.6V lead-acid battery and a couple of Chinese voltage converters are enough.
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